Due largely to the development of the small computer, there has simultaneously developed an urgent need for a printing device of simple construction which is small in size, light in weight and low in price. Simplicity in the construction makes it easy and inexpensive to replace the device, either in part or completely, if necessary. Also, the likelihood of failure is diminished and servicing is both easy and inexpensive. However, the constructions now in use suffer from a number of deficiencies which can be understood from an examination of a conventional device such as is shown in FIG. 1. As the mechanism for selecting a character on the printing type ring for printing, a printing type ring shaft 1 has a groove 2 on its periphery. The shaft supports printing type rings 5 each of which has type face characters 3 on its periphery. The rings engage frictionally with groove 2 by means of a printing type ring spring 4 for rotation in the direction of arrow a. When a desired character 3 has reached the printing position 10, a selecting pawl 8 is rotated in the direction of arrow b by a selecting electromagnet 6 and a selecting spring 7. The pawl engages with a ratchet tooth 9 on the side face of printing type ring 5, thus stopping said ring 5. Shaft 1 continues to rotate, however, and accordingly, groove 2 is so shaped that printing type ring spring 4 pulls out of groove 2 on said printing type ring shaft 1 and rides on the periphery of said printing type ring shaft 1, thereby disengaging ring 5 from shaft 1. Then, printing is effected by bringing a hammer 11 which is opposed to ring 5 at printing position 10 against printing paper 12 placed between the hammer 11 and ring 5.
This conventional device is so constructed that after printing of the selected character takes place, the shaft 1 rotates in the direction of arrow c whereupon the corresponding ring spring 4 of the selected ring 5 hooks into groove 2, groove 2 being so shaped that the ring spring 4 cannot disengage from the corresponding grooves when the rings are rotating in the c direction.
Conventional printing mechanisms use either frictional engagement between the ring and the shaft or coil springs to effect the engagement. In such constructions, when printing depends on the transfer of ink from an ink roll (not shown in FIG. 1) to the characters, and where the printing type rings 5 must stop and rotate in both directions, both sliding contact and rolling contact between the ink roll and the printing type rings 5 are effected. Under such circumstances both types of contact but, especially sliding contact, result in wear and deformation of the ink roll by the characters as a result of which the durability of the ink roll is adversely affected. A further point of great importance is that the drag on the rings produced by contact with the ink roll is substantial, as a result of which it becomes necessary to use large, heavy drive means which consume substantial amounts of power. An additional difficulty is that the drag imposed by the ink roll on the rings during selection frequently produces errors in selecting the desired characters.
These difficulties have been recognized, but the steps taken to overcome them have increased the weight and size of the mechanism. In order to overcome the tendency to err in the selection of characters to be printed, the load on the printing type ring springs 4 has been increased. However, this increases the load on the whole mechanism making it difficult to obtain sufficient functional reliability. The problem occurs and is serious when said ink roll is brought in contact with the type ring either during the selection process or in the return process. In trying to eliminate this problem any increase in the number of process steps is undesirable because the printing speed is decreased proportionately. Furthermore, in such an improved process the ink roll must not be brought in contact with the ring during the selection and return processes, so it is required that whatever mechanism be used that the ink roll be brought in contact with the rings only intermittently, but a mechanism for carrying out such a step is apt to be expensive.
As to the mechanism for turning the ring shaft alternately in both directions, in one type of device energy is stored in a torsional coil spring at the moment of reversing the direction of rotation of the shaft. This involves the use of gearing which can be disengaged at the moment of reversal of the direction of rotation, together with a mechanism for changing gears so that the mechanism is complex. The energy loss in such a mechanism is great and the wear problem is serious due to the impact of the torsional spring. Furthermore, the impact noise is great making it difficult to use such a mechanism in an enclosed space. Moreover, the accuracy of the gears used must be very high so that the gears become expensive, and even then actuation is unstable and lacks reliability.
In the mechanism shown in FIG. 1 where printing type ring springs 4 connect the ring 5 frictionally with printing type ring shaft 1, additional energy must be supplied to pull the springs 4 out of groove 2 and to slide the end of springs 4 along the periphery of shaft 1 after the rings 5 have been stopped for the printing step. Since the number of rings on a shaft may be as large as 40 or even more, the total load imposed on the drive mechanism becomes large so that the energy consumption of the whole mechanism leads to the need for large, heavy and expensive components.
As aforenoted, printing mechanisms have thus far incorporated the feature that the rings carrying the characters 3 rotate with the ring shaft 1 when they are not selected for printing and the rings 5 are stopped only when they are selected, after which resetting of the rings 5 is effected by the reverse rotation of shaft 1. Also, as aforenoted, the use of an ink roll in such a mechanism has introduced the problems of durability of the ink roll, erroneous selection of characters 3, increase in the load on the drive mechanism, serious decrease in the printing speed and increase in the cost of the mechanism itself and the products produced by such a mechanism. In the use of a reciprocating mechanism such as is shown in FIG. 1 where the mechanism depends on a printing type ring shaft 1 and pawl and groove combinations for selecting specific characters 3, the energy consumption is great whereas it is extremely desirable that the energy consumption be low, both from the standpoint of the initial cost of the components and from the cost of operation. Moreover, the reciprocating mechanism of a device such as is shown in FIG. 1 comprises a large number of parts which must be made with high precision and are consequently expensive. It would be desirable to reduce both the power consumption needed the size of the components and their cost. It would also be desirable that the device be simplified and freed of the need for expensive, high-precision components.